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An introduction is given to the fracture mechanics approach employing the concept of crack opening displacement for application to situations in which linear elastic fracture mechanics is invalidated by yielding. The hypothesis of a critical crack opening displacement to fracture has been examined experimentally using mild steel specimens of vastly differing dimensions. Subsidiary experiments have been carried out to define the factors responsible for the apparent effect of absolute size on the results. A theoretical analysis simulates elastic-plastic conditions to give a relationship between applied stresses and strains, crack length, and crack opening displacement.

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North American railroads are experiencing an increased number of axle failures. The greatest increase has been observed in unit train coal wagons that are operated with 32 400 kg axle loads. This level of loading has recently been permitted in service and represents a 10 per cent increase in the formerly permitted level. Failure statistics and the resulting increase in stress are discussed and several solutions are presented.